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• S Technical Description The different materials are always different specific gravities and are usually made up of various sizes M of gravel,garnet, sand, and anthracite coal. The-denser filter media are placed below the coarser but L less dense coal, thus allowing deeper penetration in the direction of flow and maintaining a coarse surface layer even after backwash. This filter media design concept is termed multimedia. BODS concentrations that are trapped in the suspended solids of the secondary effluent can be effectively removed in a well-designed tertiary filter;however,standard multimedia filtration has little or no measurable effect on the removal of soluble BOD which is generally 10 microns or less in size. In applications where extreme effluent polishing is required, super or microfilters may be added after the multimedia filter. Even a reverse-osmosis (RO) system may be used as a final filter. RO in conjunction with microfilters is at a state in technological development where it is possible to remove particulates down to 0.1 micron or 0.00000394 inch, although it is very unlikely the need for this type of filtration will ever be required in the wastewater treatment and reclamation area. The most important consideration in the sizing of tertiary filters, and one requiring considerable engineering judgment, is selection of the application rate, which is expressed as gallons per minute applied per square foot of filter surface area. In many cases, the maximum application rate is fixed by a regulatory agency. Some factors that must be considered in selection of the rate of application include: 1) efficiency of the secondary treatment process;2) flow pattern and; 3) filter cycle duration and backwash water production; 4) capital cost of equipment. With the Bio-Pure system, designed-in plant flow equalization allows filter flow rate selections based on equalized flow rates, which removes the problems that result from periodic surge flows. With the unique batch design of the Bio-Pure secondary process, daily flow patterns(as well as peak flow rates) are near constant, thus a set schedule can be maintained for backwashing the filter media during low system flow periods. ELECTRONICIELECTRICAL CONTROL SYSTEM The Bio-Pure system control unit provided by AquaClear manages the operational functions of the entire treatment system with solid-state memory. The memory is programmed at the factory and is retained in the program controller (PC) by either a battery backup or an EPROM. The memory L consists of firmware located within the solid-state assembly. a In addition to the PC are solid-state relays,circuit breakers, a discharge flow counter, and a settling time totalizer. Start/stop cycles of individual pumps are preprogrammed into.the microprocessor. These cycles are based upon pumping and settling times and are initially set at the factory. Actual field operation times may vary; thus, provisions are made for reprogramming the microprocessor in the field. All pump cycles are protected from memory loss by standby battery or by the EPROM,which retains the program memory during periods of electricalinterruptions. 18